Electroless plating is a proces that is well-known in the art for chemically depositing metal on metallic or non-metallic substrates. The electroless plating of copper is of considerable commercial importance in the electronics industry, particularly in the fabrication of printed circuitry used in electronic equipment. In fabricating printed circuitry, a thin layer of copper must first be deposited by electroless plating onto a non-conductive surface such as plastic, to make the surface electrically conductive. This thin conductive surface serves as an electrically conductive flash layer receptive to further deposition of metal by electro-deposition, which is used to build up a plate to a desired thickness for an actual conductive circuit pattern.
The typical prior art electroless plating process generally comprises the steps of treating the surface of the article to be plated by immersing the article in a bath containing a stannous salt; catalyzing the article by immersing it in solution for providing catalytic nucleating centers on the surface of the article; and treating the catalyzed surface of the article by immersing it in an electroless solution including a salt of the metal and a reducing agent. The steps of the electroless plating process typically are performed in a tank that is filled with a static bath corresponding to the desired step of the electroless plating process. The article to be plated, such as a printed circuit board substrate, is immersed in the static bath until the desired treatment is completed.
In the prior art, the process steps are performed sequentially by draining the tank and refilling it with the constituent bath of each succeeding step until the entire process is completed. Alternatively, the process may be performed by maintaining a series of tanks and sequentially immersing the article to be plated in each tank containing the appropriate bath.
There are several serious drawbacks to using the above-described prior art static systems of electroless deposition. First, static systems are characteristically slow. For example, it can take longer than a day to plate a single printed circuit board substrate with only one mil of copper by electroless deposition where static baths are used. This seriously inhibits the volume production of plated articles, such as is demanded in the commercial manufacture of printed circuit boards.
Second, the chemical constituents in electroless plating baths are continuously being consumed. Thus, these baths are in a constant state of change. It is extremely difficult to control the chemical stability of a constituent bath to maintain a relatively high plating rate over long plating periods as required when prior art systems are employed. Consequently, the static baths tend to become unstable and decompose with use. Therefore, chemical stability, which is important in order to maintain a relatively high plating rate for the duration of the plating process, is lost.
Third, with respect to the plating of through-holes, the quality and speed of electroless plating of tiny through-holes in printed circuit boards is severely limited by prior art electroless deposition systems. Typically, in printed circuit board fabrication, electroless metal is deposited as a uniform surface coating or in a predetermined pattern on a nonconductive substrate. The substrate is generally copper-clad plastic laminate, having a copper foil laminated to one or both substrate surfaces. In double-sided printed circuit boards, and in multilayered printed circuit board packages, connections are provided between conductive surfaces by means of through-holes drilled in the laminate. The walls of the through holes are made conductive with an electroless coating. Such holes, because of their minute size, can be very difficult to access with the constituent baths of the electroless process using a system of immersion in a static baths. Consequently non-uniform or incomplete electroless plating of through-holes may result. This significantly reduces the quality of the printed circuit board produced, and increases the number of boards that ultimately must be rejected.
Also, in prior art systems hydrogen gas produced during the plating process may accumulate in the through-holes, as well as in tiny pits and pores in the printed circuit board surface, which further interferes with the electroless plating process. Additionally, because of the resulting amount of included hydrogen in electroless metal deposits, the plate may exhibit poor ductility, and therefore easily can be fractured in later use by vibration or bending.
Accordingly, it is a primary object of the present invention to provide a new and novel method and apparatus for electrolessly plating articles which substantially increases the reaction rates of the constituent electroless plating baths, and that deposits a given thickness of metal onto a substrate in a fraction of the time required in prior art systems.
Another object of the present invention is to provide a method and apparatus for electroless plating that provides complete and uniform plating of apertures in the articles to be plated, such as in printed circuit board through-holes.
It is an additional object of the present invention to provide a method and apparatus for electroless plating that results in a uniform and ductile metallic layer on the entire surface of the article, such as on both side of double sided printed circuit boards.